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PTHREAD_CREATE(P) PTHREAD_CREATE(P)
NAME
pthread_create - thread creation
SYNOPSIS
#include <pthread.h>
int pthread_create(pthread_t *restrict thread,
const pthread_attr_t *restrict attr,
void *(*start_routine)(void*), void *restrict arg);
DESCRIPTION
The pthread_create() function shall create a new thread, with attributes specified
by attr, within a process. If attr is NULL, the default attributes shall be used.
If the attributes specified by attr are modified later, the thread’s attributes
shall not be affected. Upon successful completion, pthread_create() shall store the
ID of the created thread in the location referenced by thread.
The thread is created executing start_routine with arg as its sole argument. If the
start_routine returns, the effect shall be as if there was an implicit call to
pthread_exit() using the return value of start_routine as the exit status. Note
that the thread in which main() was originally invoked differs from this. When it
returns from main(), the effect shall be as if there was an implicit call to exit()
using the return value of main() as the exit status.
The signal state of the new thread shall be initialized as follows:
* The signal mask shall be inherited from the creating thread.
* The set of signals pending for the new thread shall be empty.
The alternate stack shall not be inherited.
The floating-point environment shall be inherited from the creating thread.
If pthread_create() fails, no new thread is created and the contents of the loca-
tion referenced by thread are undefined.
If _POSIX_THREAD_CPUTIME is defined, the new thread shall have a CPU-time clock
accessible, and the initial value of this clock shall be set to zero.
RETURN VALUE
If successful, the pthread_create() function shall return zero; otherwise, an error
number shall be returned to indicate the error.
ERRORS
The pthread_create() function shall fail if:
EAGAIN The system lacked the necessary resources to create another thread, or the
system-imposed limit on the total number of threads in a process
{PTHREAD_THREADS_MAX} would be exceeded.
EINVAL The value specified by attr is invalid.
EPERM The caller does not have appropriate permission to set the required schedul-
ing parameters or scheduling policy.
The pthread_create() function shall not return an error code of [EINTR].
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
None.
RATIONALE
A suggested alternative to pthread_create() would be to define two separate opera-
tions: create and start. Some applications would find such behavior more natural.
Ada, in particular, separates the "creation" of a task from its "activation".
Splitting the operation was rejected by the standard developers for many reasons:
* The number of calls required to start a thread would increase from one to two
and thus place an additional burden on applications that do not require the
additional synchronization. The second call, however, could be avoided by the
additional complication of a start-up state attribute.
* An extra state would be introduced: "created but not started". This would
require the standard to specify the behavior of the thread operations when the
target has not yet started executing.
* For those applications that require such behavior, it is possible to simulate
the two separate steps with the facilities that are currently provided. The
start_routine() can synchronize by waiting on a condition variable that is sig-
naled by the start operation.
An Ada implementor can choose to create the thread at either of two points in the
Ada program: when the task object is created, or when the task is activated (gener-
ally at a "begin"). If the first approach is adopted, the start_routine() needs to
wait on a condition variable to receive the order to begin "activation". The sec-
ond approach requires no such condition variable or extra synchronization. In
either approach, a separate Ada task control block would need to be created when
the task object is created to hold rendezvous queues, and so on.
An extension of the preceding model would be to allow the state of the thread to be
modified between the create and start. This would allow the thread attributes
object to be eliminated. This has been rejected because:
* All state in the thread attributes object has to be able to be set for the
thread. This would require the definition of functions to modify thread
attributes. There would be no reduction in the number of function calls required
to set up the thread. In fact, for an application that creates all threads
using identical attributes, the number of function calls required to set up the
threads would be dramatically increased. Use of a thread attributes object per-
mits the application to make one set of attribute setting function calls. Other-
wise, the set of attribute setting function calls needs to be made for each
thread creation.
* Depending on the implementation architecture, functions to set thread state
would require kernel calls, or for other implementation reasons would not be
able to be implemented as macros, thereby increasing the cost of thread cre-
ation.
* The ability for applications to segregate threads by class would be lost.
Another suggested alternative uses a model similar to that for process creation,
such as "thread fork". The fork semantics would provide more flexibility and the
"create" function can be implemented simply by doing a thread fork followed immedi-
ately by a call to the desired "start routine" for the thread. This alternative has
these problems:
* For many implementations, the entire stack of the calling thread would need to
be duplicated, since in many architectures there is no way to determine the size
of the calling frame.
* Efficiency is reduced since at least some part of the stack has to be copied,
even though in most cases the thread never needs the copied context, since it
merely calls the desired start routine.
FUTURE DIRECTIONS
None.
SEE ALSO
fork() , pthread_exit() , pthread_join() , the Base Definitions volume of
IEEE Std 1003.1-2001, <pthread.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating
System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C)
2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The
Open Group. In the event of any discrepancy between this version and the original
IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is
the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
POSIX 2003 PTHREAD_CREATE(P)
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